Conventionally, a technique for detecting the air pressure of a vehicle tire, and informing the user of the detected air pressure (to be referred to as a measured air pressure hereinafter) as a tire air pressure state is known.
Note that a normal user recognizes a normal air pressure based on the description of a sticker or the like, which is adhered in the vicinity of a vehicle door or the like. In such description, the normal air pressure is that at ambient temperature regardless of the tire inner temperature.
On the other hand, since the tire inner air temperature varies under various influences such as a change in temperature around a vehicle, a temperature rise of a rim upon braking during traveling, and the like, the tire inner air pressure also varies in correspondence with variations of the tire inner air temperature.
Therefore, since the conventional description of the sticker or the like indicates only the measured air pressure that does not consider any tire inner temperature, it is difficult for the user who recognizes only the normal air pressure at ambient temperature to determine if the indicated air pressure is higher or lower than the normal air pressure.
Hence, Japanese translation of PCT publication No. 10-508264 discloses a technique for correcting the air pressure signal of a tire detected by a pressure sensor in association with a standard temperature (20° C.), and displaying the corrected air pressure signal on a display of a vehicle.
However, in the aforementioned prior art, the display of the vehicle and an external air pressure gauge of an air pump or the like indicate different air pressures, and the user may become confused.
More specifically, in the aforementioned prior art, when the air pressure, which is corrected in association with the standard temperature, is lower than a normal air pressure, the display of the vehicle displays that lower air pressure. For this reason, the user may pump air in a tire to raise the air pressure to a normal value according to the displayed contents.
However, when the tire temperature is high, although the air pressure displayed on the display of the vehicle is low, the actual air pressure becomes higher than the normal air pressure due to the high tire temperature in such case, and an air pressure gauge of an air pump indicates an air pressure higher than the normal air pressure. That is, since the display of the vehicle and the air pressure gauge of the air pump indicate different air pressures, the user may wonder whether or not to pump in air.